1. Field of the Invention
This invention relates to the biological treatment of water and, particularly, to a secondary treatment process which includes an aerobic or anerobic reaction. More specifically, the present invention is directed to filtration apparatus wherein a liquid feed stream is caused to flow over the surfaces of a granular medium in the presence of a gas necessary for a biological purification reaction and, especially, to a filter wherein the medium on which microbes grow is in the form of a fluidized bed of granules separated by a sieve plate from a clean water collection zone. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
The biological purification of water is, of course, of great importance and ecological significance. Thus, by way of example, biological reactions are commonly employed in the purification of municipal and/or industrial waste water in order to produce service water and drinking water. Biological purification is customarily used as a secondary treatment, i.e., the waste water delivered thereto will have been subjected to pre-purification. Such pre-purification may be accomplished in collecting or preclarifying tanks. The secondary treatment is often done in a filter vessel to which the waste water from the primary treatment stage is directed. As the waste water flows through the filter vessel it will react with an oxygen rich gas and an aerobic reaction will occur. In such a reaction, the bacteria present metabolize biodegradeable organics in the waste water to produce microbial solvents which are removed by gravity sedimentation. Alternatively, rather than creating an environment with excess dissolved oxygen, biodegradeable organics in the waste water may be converted into suspended organics which flocculate by means of an anaerobic reaction. In either case, to produce an acceptable quality effluent, the filter vessel must contain a material which defines a porous filter having sufficient surface area for microbe growth.
European patent document EP-B-0 227 081 discloses a prior art trickling filter type device for use in water purification. In the device of this publication, the water to be purified is conducted through a closed trickling filter in the presence of a pressurized, oxygen enriched atmosphere whereby the trickling filter functions as a biofilm support. The effluent from the trickling filter passes into a "floating-grain" pressure filter. In the floating grain pressure filter, clean water collects above the floating granular filter material, the upper limit of the bed comprising the floating filter material being limited by a sieve plate. An air cushion is established above the clean water which has passed through the floating-grain filter. In the apparatus of the referenced European patent document, oxygen required for biofiltration in the pressure filter is absorbed by the untreated water in the trickling filter with the result that oxygen consumed in the biofiltration process cannot be replaced. Also the floating grain is cleaned by causing the air cushion to periodically force the clean water downwardly through the floating grain pressure filter. Restated, reverse flow pulses of clean water are forced through the floating grain pressure filter to wash the oxidized, i.e., solid waste, material off the granules of the bouyant filter medium. The waste material washed off the floating grain is collected in a sludge thickener which is situated beneath the trickling filter. The periodic pulsing of the clean water, i.e., the shock-like washing of the floating grain, interrupts the biological process. After each such interruption, the requisite gas pressure within the trickling filter must again be established. Further, the process of the referenced European patent document requires complex apparatus including at least two connected pressure vessels and at least two sieve plates.
Published German patent document DE-C-3 338 170 discloses a filter vessel containing a floating, granular filtration medium wherein the liquid to be purified flows from the top to the bottom of the vessel during the biological purification reaction. In the technique disclosed in this German publication, the cleaning of the filtration medium is carried out in a separate washing vessel which is connected to the filter vessel and disposed at a lower level than the filter vessel. Granules of the filter medium are removed from the filter vessel at the top end thereof and pass into the washing vessel as a result of a pressure differential maintained between the filter and washing vessels. In the washing vessel, the filter granules are washed with clean water. The washed filter material is then allowed to flow back into the filter vessel as a consequence of its buoyancy. The apparatus disclosed in this German patent document is expensive to manufacture and, because the floating grain filter medium is washed in batches, its utilization requires the implementation of relatively complicated and expensive control procedures.